Xylenes separation



Nov. 7, 1950 HEN ETAL 2,528,892

XYLENES SEPARATION Filed Aug. 25, 1948 3 Sheets-Sheet 1 PARTIAL PRESSURE OF BF; IN INCHES'OF Hg I I MG, I I O I .L- on one 004 0.06 one; 5 010 0.12 MOL FRACTION BF; IN If A I p o 05 m a 5 ol 1 i 5 MOL RATIO 0 0.5 L0 BF; TO XYLENE m I l I l i mmvrozas: Arthur R Lien FIG 1 8 David A. McCau/ay Alla/nay Nov. 7, 1950 A. P. LIEN EI'AL 2,528,892

xnms ssmm'mn Filed Aug. 25, 19.48 3 Sheets-Sheet 2 I I I 70 3 o Q a 5/ 8/ U as 40 i I PARTIAL PRESSURE 0F BF; m INCHES 0F. Hg.

I I I 1 I w j O MOL FRACTION BF; IN HI? 05 10 MOL RATIO: BF; TO TOLUENE 1 MOL RATIO: BF; TO ETHYLBENZENE zm mroxs:

Arthur I I? Lien FIG. 2. 8 David A. MgGau/ay Adm/$ 0M Nov. 7, 1950 A. PQLIEN .ErAL 2,528,892

' ms'sifrm'rron Filed Aug. 25, 1948' 3 Sheets-Shoot 3 mVmrong: Arthur R Lian Alia/nay David A. "(2000/01 7* Patented Nov. 7,1950

XYLENES SEPARATION Arthur P. Lien, Hammond, Ind., and David A. McCaulay, Chicago, Ill., assignors to Standard Oil Company, Chicago, 111., a corporation of Indiana Application August 25, 1948, Serial No. 46,136

8 Claims.

This invention relates to a process for .the selective separation or concentration and recovery of certain dialkylbenzenes from their mixtures with other dialkylbenzenes, which mixtures may also contain monoalkyl benzenes, benzene and saturated hydrocarbons. In one specific form, this invention relates to a process for the selective separation or concentration and recovery of individual xylene isomers from narrow boiling range aromatic hydrocarbon fractions containing the same. This invention relates more particularly to a process for the selective separation or concentration of meta-xylene from narrow boiling range aromatic hydrocarbon fractions containing meta-xylene, para-xylene, ethylbenzene and saturated hydrocarbons boiling in the boiling range of the xylene fraction and normally associated with said xylenes as produced by hydrocarbon conversion processes; the aromatic hydrocarbon fractions may also contain ortho-xylene.

In accordance with this invention, we have observed that by the joint employment of liquid HF and BF3 as a selective reaction solvent under appropriately selected operating conditions, it is possible to achieve the selective extraction of certain meta-dialkylbenzenes from other dialkylbenzenes and less-alkylated benzene hydrocarbons.

We have also made the surprising observation that ethylbenzene, which is usually associated with xylenes as produced in hydrocarbon conversion processes, undergoes disproportionation with surprising ease in the presence of liquid HF and BF3 to produce chiefly benzene and dlethylbenzenes and, in the presence of xylenes, ethylxylenes. The disproportionation of ethylbenzene is of immense value in simplifying its separation from xylenes and makes it possible thereafter to separate selectively or to concentrate individual xylene isomers from each other by means of liquid HE and BF3, without interference by ethylbenzene.

It has been appreciated heretofore that liquid HF and BF; function jointly as a solvent or solvent-catalyst for aromatic hydrocarbons generally, presumably by the formation of complex compounds wherein HF, BF; and an arcmatic hydrocarbon are joined and dissolved in excess HF and BFE; (U. S. Patents 2,343,744; 2.343341 and 2,405,995, for example). However, it was not heretofore appreciated that liquid HF in combination with BFs could be employed in such a manner as to exert selective solvent characteristics for certain aromatic hydrocarbons in heretofore been revealed nor has any publication been devoted to the relative stability of the various aromatic hydrocarbon complexes with liquid HF-BF3. It appears to have been known with certainty only that if the postulated complexes of HF and BF: with aromatic hydrocarbons did exist, they all appeared to be uniformly and highly soluble in excess IE or HF-BFz,

- so that HF and BE; appeared to oflfer no promise as a selective solvent for the extraction of one aromatic hydrocarbon from another.

We have made observations which lend weight to the postulations concerning aromatic hydrocarbon complexes with BF% and have studied the composition and properties, especially the relative stabilities of various such complexes. Surprising variations were observed in the stabilities of BFz-isomeric xylene complexes which were unexpected because of the close chemical and physical similarities of the xylene isomers. We

. have also observed that the relative stabilities of aromatic hydrocarbon complexes containing BF: can be substantially correlated with the actual extractabilities of the individual aromatic hydrocarbons by liquid HF and BFa.

Although benzene and toluene have been stated to form complexes with liquid HF and B35, we have observed that such complexes, if they are formed at all, are completely dissociated (i. e. they do not exist as such) at temperatures as low as 0 C. e

We have also found that isomeric xylenes may be substantially completely extracted frommixtures containing the same and saturated hydrocarbons of close boiling range by contacting said mixtures with a molar excess of liquid hydrogen fluoride and with BF: in at least an equimolar ratio, based upon the aromatic hydrocarbon content of said mixture. As has been pointed out above benzene and toluene do not appear to form complexes with HF and B35, with the result that xylenes may be selectively ex- 3 tracted from mixtures'thereof with benzene and toluene (as well as saturated hydrocarbons) by the employment of liquid HF and BF: in an amount which is at least equimolar with respect to the xylenes present in said mixture.

Also, when a solution of xylenes in liquid HF and BF; is treated to withdraw suflicient BFs to reduce its concentration below 1 mol per mol of aromatic hydrocarbons contained in said solution, the solution can be readily separated into two layers by conventional means such as settling, centrifuging or the like. Analysis of the two resultant layers indicates that the xylene isomers are distributed in different proportions in the two layers, viz., the raflinate layer which is characterized by containing only small amounts of HF and BE! and which consists essentially of complex-free xylene isomers and the extract layer which consists of BFa-HF-xylene isomers dissolved in excess liquid HF. The distribution of xylene isomers in the raflinate and "extract layers is essentially the same as would be produced by initially contacting the xylenes with a molar excess of liquid HF and with BF; in an amount less than 1 mol per mol of xylenes. The withdrawal of BF: from the solution containing xylenes can be effected by various methods which will be pointed out in some detail hereinafter.

One object of our invention is to provide processes for the selective separation or concentration of individual xylene isomers from mixtures of xylene isomers. An additional object of our invention is to provide processes for the selective separation or concentration and recovery of meta-xylene from mixtures thereof with paraxylene or orthoxylene or from mixtures containing all three isomeric xylenes. Yet another objectof our invention is to provide a process for the separation of a mixture of xylenes or concentrates of individual xylene isomers from ethylbenzene.

A further object of our invention is to provide a process for the separation of xylenes from mixtures thereof with benzene and monoalkylbenzenes, particularly toluene and ethylbenzene. Still another obiect of our invention is to provide a process for the separation of a mixture of isomeric xylenes from associated, close boiling saturated hydrocarbons, following which individual xylene concentrates are produced by the process of this invention.

Another important object of our invention is to provide a practical process for the concentration of individual xylene isomers from xylene fractions such as are normally produced by hydrocarbon conversion operations or by the extraction of petroleum fractions. The above and other objects of our invention are achieved by the process whose details are set forth below.

Since the extraction processes involved in our invention ap ear to function through the formation of BFa-HF-arnmatic hydrocarbon complexes, detailed consideration will be given to the identities and properties, especially the relative stabilities of such complexes. In order to prepare and study the com lexes in question, a BF; storage cylinder, a BFb metering flask and a reaction flask were valved to a manifold of copper tubing which was suitably attached to an evacuation pump. A pressure gage was also attached to the copper manifold line.

A weighed amount of HF and aromatic hydrocarbon (usually about 5 moles of HF and about 0.4 mol of alkylbenzene) were introduced into the reaction flask. The flask was then attached to a vacuum system, its contents frozen'with liquid nitrogen, the whole system evacuated and the valve to the vacuum pump closed. Boron trifluoride was then passed into the system from the storage cylinder and allowed to flll the meterin flask. The valve of the'reaction flask was then opened and increments of BF: were condensed in the flask. The valve of the metering flask was then closed and the reaction flask allowed to warm to 0 C. in a water-ice bath. The flask was agitated at 5-minute intervals at this temperature until a constant pressure was reached; this usually required from one to two hours. After the pressure reading was taken, the process was repeated, 1. e., the contents of the flask were again frozen down in liquid air and another increment of BF: was added and the pressure was read again when the system came to equilibrium.

The following data were obtained employing the above procedure at 0 C., on the liquid HF-BFa-system (containing no hydrocarbons).

TABLE 1 BFs-HF system g. BF: in liq. BF: Mol Partial l Press. HF

phase HF Fraction of g f (in. Hg)

0 10s 0 0 l5 2. 8 106 0079 11. 5 15 5. 0 106 0141 33 15 p 1. 2 10s .0200 54. s .15

about 0.25% water in the HF. Neglecting this small deviation it is seen that BF; in HF obeys Henry's law, which at 0 C. may be expressed as P1ar =3530N1ar where Pnr' is the partial pressure of BF; in inches of mercury and N3 is the mol fraction of BF3 in solution in liquid HF.

These results show that the solubility of BF: in

liquid I-lLF is relatively low. At atmospheric pressure and at 0 C. about 3.5 g. of BFa will dissolve in\ g. of HF. There is very little tendency for HF and BFa in a binary system to combine to form the often postulated compound HIBF4.

The following data were obtained in a study of the system BFa-flF-meta-xylene at 0 0., employing 44 g. (0.415 mol) of meta-xylene and 93.5 g. (4.675 mols) of liquid HF in the reaction flask.

TABLE 2 BF3-HI-"-1neta-zzylene system MQIS i \riol tactic: To? Press P81211811; lg ress. Pal-gs}; HF 50111. Hg) (in. Hg) (in. Hg)

The partial pressure of BFa was plotted against the mol fraction of BF: in the HF as shown in Figure 1. A comparison of the partial pressure of BF: over the HF-BFa-meta-xylene system with the HF-BFE system shows that the inclusion of meta-xylene has resulted in a tremendous reduction 01' the vapor pressure of BB, clearly indieating the formation of a BFa-HF-meta-xylene complex which has a relatively low vapor pressure compared to HF-BFa. It will be noted that the Per curve approaches an asymptote which is almost parallel to the PBF3 curve of the HF-.BF3 system. Extrapolating the asymptote to PBF3=0 indicates the composition of BFa-HF-metaxylene complex, which can be read off the scale indicating the mol ratio 01' BFa-meta-xylene. It will thus be seen that the meta-xylene complex contains one mol or BF: per mol of meta-xylene. This complex of meta-xylene probably also contains one mol of -HF and the complex is dissolved by the excess liquid HF which is present in the reaction flask. It appears that the following reaction takes place in the reaction flask:

BFa+fiF-l-meta-xylenez BFa.meta-xyleneHF The equilibrium constant for this reaction can be defined as follows:

where,

From the standpoint of calculating equilibrium constants from the data, a more practical expression of the above equation is Keq=mols HF wherein N'rer is the total mol fraction of BF: in the liquid phase, including both free BF: and BF; combined in the hydrocarbon complex. N'rsr' can be determined from the PBFS vs. BF: mol fraction curve (Figure 1). New is determined from the Henry's law expression (supra). Nnr can be approximated by subtracting N'riarfrom one.

A study of the vapor pressure of the system meta-xylene-BFa-HF over the range of C. to 25 C. showed that the following equation defines the variation in the magnitude of the equilibrium constant with temperature.

Log Keq= ,i. 2.434

From this equation it is seen that, for the reaction BF: (gas) +meta-xylene (1)+HF (1) BFz.meta-xylene HF (1), A H=(2.303 R) (642) =2.9 K cal. This value of A H is approximately the same as the heat of condensation of BFs.

The following data were obtained in a study of the system BFs-HF-ortho-xylene at 0 (3., cmploying 44 g. (0.415 mol) of ortho-xylene and 99.5 g. (4.975 mols) of liquid HF.

TABLE 3 BFa-HF-ortho-xylene-system The partial pressure of BFa was plotted against the mol fraction of BFa in the HF, as was done in the case of the meta-xylene complex (shown in Figure 1) and a curve was obtained which shows 5 that or'tho-xylene likewise formed a. complex containing 1 mol of BFa per mol of ortho-xylene. However, a comparison of the'paltial pressure of BFa over the meta-xylene and ortho-xylene complexes shows that the ortho-xylene complex is much less stable than the meta-xylene as evidenced by the far greater Pmover the orthoxylene complex.

The following data were obtained in a study of the system BFg-llF-para-xylene at 0 C. employl5 ing 41 g. (0.387 mol) of para-xylene and 109.5

g. (5.475 mols) of liquid HF.

TABLE 4 BFs-HF para-xylene Wt. BF: added Total Press.

Mols BFz (inches Of in liq.

Mol fraction Partial Press. BF: in liq.

Keq Ratios para-xylene TABLE 5 BFr-HF-toluene system at 0 C.

r1015 BFain Mol fraction Total Press. P

23 liquid BFa in liq. (in. Hg) 2&

When the mol fraction was plotted against the partial pressure of BF: (Figure 2) a straight line drawn through the points passed close to the origin and had a slope almost as steep as the line for HF-BF3 system. This means that HF, BFs, and toluene have very little, if any, tendency toward complex formation even under high BF; pressures. It follows,"therefore, that liquid HF and BF; can be used as a solvent for the selective extraction of the xylenes from toluene.

tion had the following properties: n =1.4954,

B. P.=136 C. A portion of this material was used in the vapor pressure measurements, employing 44 g. (0.415 mol) and 114.5 g. (5.725 mols) of HF.

TABLE 6 wear, Total Press. M018 BF in Molflacfion Partial added at 0. Press. ofBF; (in. oi Hg) 3 (in. Hg)

1 Pressure was 55" Hg at start. 0n shaking for 15 minutes at 0 C. it gradually fell to 17.5 Hg.

The mol fraction of BFa was plotted against its partial pressure (Figure 2). The plot showed that a complex was formed having the same order of stability as the BFa-HF-meta-xylene complex. However, the plot also showed that only 0.207 mol of BF: were required to form the complex, which is exactly the number of mols of ethylbenzene present. This suggests that ethylbenzene disproportionates according to the following equation:

and forms a diethylbenzene, which forms a. complex with BF; and HF, and benzene which does not form a complex with BF% and HF.

A fractionation analysis of the hydrocarbon recovered from the experiment showed it to have the following composition:

Vol. Per M01 Per Cent Cent 28 40 Ethy 5 5 Diethy 67 55 Evidently, under the conditions of the experiment, the ethylbenzene disproportionated almost quantitatively into equimolar amounts of benzene and diethylbenzene, and it was the latter which formed a monomolecular complex with the BF: and HF.

Suitable feed stocks for our selective extraction process are mixtures of xylenes. The boiling points and freezing points of the xylenes, and of ethylbenzene which boils within the same range,

are

B. P., "F. F. P., F.

ortho-xylene 291. 95 -l3. 32 meta-xylene 282. 38 -54. 17 para-xylene 281. 03 +55. 87 ethylbenzenm. 217. 14 138. 96

(Selected Values of Properties of Hydrocarbons"-Nat. Bur. Btds.O461-Nov. 1947-p. 67).

From the above boiling point data it will be apparent that a mixture of metaand paraethylbenzene.

xylenes cannot be resolved by fractional distillation and this is essentially true also of a mixture of either metaor para-xylene with Although 'ortho-xylene can be separated by superfractionation from the isomeric xylenes and ethylbenzene, such a process entails extremely expensive and large equipment which, because of the high reflux ratios which are necessary, has only a low throughput. Also, it is practically impossible to obtain an orthoxylene concentrate containing more than about 70% ortho-xylene by superfractionation from xylene fractions containing saturated hydrocarbons boiling in the same range, due to the formation of azeotropes of overlapping boiling ranges of the various xylenes with the saturated hydrocarbons. Para-xylene has been separated from mixtures thereof with meta-xylene by fractional freezing, which is an expensive and laborious procedure compared with the process of the present invention. Fractional freezing or melting is severely handicapped, even disregarding economic considerations, by the fact that paraand meta-xylenes form a eutectic mixture containing about 88 weight percent metaand 12 weight percent para-xylene (M. P., -73 F.) and para-xylene can not be selectively frozenfrom mixtures containing less than about 16% paraxylene (U. S. Patents, 1,940,065 and 2,398,526 and British Patent 585,076).

Aromatic hydrocarbon charging stocks suittable for employment as feed stocks in the process of this invention can be prepared by a variety of processes, probably the most important of which is the catalytic hydroforming process. For the preparation of xylene-rich products, a desirable charge to hydroforming is a light naphthenic naphtha rich in dimethylcyclohexanes (boiling range about 230 to 280 F.) In this process a petroleum naphtha, which may be a Virgin or cracked naphtha or mixture of both, is converted to aromatic hydrocarbons by contact with a solid, porous dehydrogenation catalyst at a temperature in the range of about 850 F. to about 1050 preferably in the presence of hydrogen. Suitable catalysts are oxides of metals of groups 2 to 6 of the periodic system, particularly oxides-of 6th group metals such as chromium and molybdenum, preferably supported by alumina or magnesia. Excellent catalysts can be prepared by depositing about 4 to about 10% of molybdenum oxide upon an activated alumina. Suitable space velocities for hydroforming fall within the range of about 0.2to about 4 volumes of the liquid charge per hour per volume of catalyst space. About 0.5 to about 8 mols of hydrogen can be charged to the process with each mol of naphtha feed stock. Note also, G. Armistead, Jr., Oil & Gas J. 45, 17 (Aug. 31, 1946) pp. -7 and L. R. Hill et al., Trans. Am Inst. Chem. Eng. 42, 4 (Aug. 25, 1946) pp. 611-637.)

Other sources of xylenes and other aromatic hydrocarbon charging stocks for our selective solvent extraction process are catalytic cracking, catalytic dehydrogenation of naphthenes over dehydrogenation catalysts such as NiS-WS2 or the like, thermal cracking at high temperatures, preferably in the presence of steam, for example as in the Forward process; the extraction or extractive distillation of virgin naphthas and kerosenes with selective solvents such as phenol, furfural, methanol, ethylene glycol, S02 and the like; the coking of coal, which yields a light aromatic oil from which it is conventional practice to produce nitration grade xylenes, etc.

We may also employ mixtures of isomeric xylenes obtained by the isomerization of xylenes with liquid HF-BFa, various Friedel-Crafts type catalysts, clays and the like. The above-menstood that this example is merely an illustrative embodiment carried out on a small scale through a single extraction stage.

tioned processes for the preparation of aromatic 5 Example hydrocar o suitable as charging Stocks for A batch extraction experiment was carried out 0111' Selective Solvent extractiqn process are in a 1570 cc. carbon steel autoclave fitted with a trative O ly. and it Should be understood that 1725 R. P. M. mechanical stirrer. A sample of O r Process is genera-11y applicable to the selec a C-8 cut of hydroformer xylenes was introtive extraction of meta-dialkylbenzenes from iso- 1o duced into the reactor along with liquid HR meric dialkylbenzenes regardless of the specific Next 11 was tt from a n weighing method by which the charging Stock was cylinder and the mixture was stirred for one-half D hour at 68-77" F. The initial reactor pressure was Suitable Xylene mixtures which be used as 350 p. s. i. g., but when stirring was started the char in stocks in ou process a been e pressure immediately fell to o p. s. i. g., showing lyzed by A. D. Streifi and F. D. Rosslni. These 111- that the 21 was absorbed At the end of the vestigators have reported the results of analysis, Stinqng period the mixture was allowed to settle. by measurements of freezing points of fl and the two phases were separated. The HF and t mixtures- Supplemented by analytical distll' BFswere removed from the extract phase by lation. of the four individual CB alkylbenzenes vacuum distillation and the composition of the (ethylbenzene, OIthO-XYIene meta-xylene and extract, as well as the composition of the raflinate para-Xylene) Occ in the Product from the and the feed were determined by fractionation followin fi di nt catalytic petroleum followed by ultra-violet absorption analysis. The fining processes: (1) Hydroforming, (2) Tworesults of the experiment are shown in Table 7. pass Fixed Bed catalytic cracking, (3) Three- The diflference in composition between the ratpass Fixed Bed catalytic cracking, (4) -Lowfinate and extract shows that meta-xylene was temperature Fluid catalytic cracking, and (5) Selectively extracted y the 3- High-temperature Fluid catalytic cracking. The data indicate that the relative amounts by TABLE 7 volume of the four Cs alkylbenzenes are not greatly different in the five diflerent products, being, Batch fi; xylenes on the average, as follows: ethylbenzene, 12; ortho-xylene, 21; meta-xylene, 48; para-xylene React!" Charge 19%. These amounts correspond substantially to Hydmmrmcr xylenes 239 (225 mols) those called for in chemical thermodynamic equi- 1 9 g (L4 mols) librium for the operating temperatures involved. HF 354 g, (17,7 1 The naphthenic plus paraifinic hydrocarbon content of the samples varied from about 7 to about Results percent by volume. (Alkylbenzenes in the 40 Total hydrocarbon recovery=94 wt.

Ultra-violet Absorption Analysis, Wt. Per Cent Wt. (g.) aif 2%.; iyiii. $325.11. Total Feed 239 100 19. s 41. 2 16.7 19.7 91. 2 Raffinate---- 95. a 42. 5 26. 1 11. 1 2a. a 26. 1 9m Extract 12s. 6 51. 5 19. 4 so. 1 12. 5 o. 1 92. 1

Individual Hydrocarbon Balance, Wt. Per Cent ie 1 1e fi s Ethylbemene 19. c 41. 2 l6. 1 19. 1 22. s 43. 2 11. 11.2 +2.1 +2. 0 +0. 3 -s. s

Fractionation Analysis 0! Products, Volume Per Cent Raflinate Extract Bpnvene 2 0 0-8 Aromatics 98 83 0-10 Aromatics 0 17 C8 Fraction from Five Different Catalytic Petroleum Refining Processes, J. Res. Nat; Bur. Standards 39 (Oct. 1947) pp. 303-308.) I

The following example represents one practical application of the principles of this invention which were set forth above. It should be under- From the data presented in Table 7 it will be noted that 57.5% of the feed, which is equivalent to 1.3 mols of xylenes, dissolved in the HF-BFs and that 93 g. or 1.4 mols of BF3 were used, which within experimental error is equal to the mols of xylenes present in the extract phase.

'naphthas or the like (hydroformate) traction process can be readily made.

The single stage separation (enrichment) factor, a, 15

' where N and N are the mol fractions of metaxylene in the rafflnate and extract respectively. The a corresponds, in vapor-liquid distillation terminology, to the relative volatility. This is a very high separation factor and it can be calculated by the use of a McCabe-Thiele graphical analysis that a system of only four stages is needed to separate the hydroformer xylenes into an overhead product comprising 95% oand piwlenes and a bottoms product containing 95% m-xylene. The distribution of orthoand paraxylenes obtained in the present extraction are consistent with a values of 0.69 and 0.47. The a ratios of meta-, orthoand para-xylenes are, therefore, 15 .3:1.47:1, which is considered to be in good agreement with their stability equilibria ratios of 20:2:1 (supra) as determined by vapor pressure measurements.

The individual hydrocarbon balance in Table '7 shows, within experimental error, that all three xylenes remained unchanged during the run; 1. -e., they neither isomerized nor disproportionated. However, there is a net disappearance of ethylbenzene and the results of the fractionation analyses of the products shows that the ethylbenzene tended to disproportionate to form benzene plus either diethylbenzenes or ethylxylenes.

Thi disproportionation reaction simplifies immensely the xylene separation problem since ethylbenzene is thereby removed far from the boiling range of the xylenes. It will be noted that benzene was insoluble in the HF-BF; mixture and appeared in the railinate phase, whereas the C aromatic hydrocarbon disproportionation product was extracted by the HF-BF3.

An illustrative flow diagram concerning the extraction processes of this invention is presented in Figure 3. The operation of the flow diagram will be described with reference specifically to the separation or concentration of a mixture of isomeric xylenes, although it will readily be understood that the equipment is adaptable for the separation or concentrationof other dialkylbenzenes. We may charge, for example, an aromatic fraction obtained by catalytic hydroiorming of The aromatic fraction from hydroformingmay include not only the xylenes and ethylbenzene but also benzene, toluene and associated saturated hydrocarbons, i. e., paraflinic' and cycloparaflinic hydrocarbons boiling in the same range. The aromatic fraction may also contain small proportions, e. g., up to about 10 percent by volume, of Co aromatics, predominantly isopropylbenzene. The boiling range of the aromatic fraction from hydroforming may be about 170 to about 325 F.

The aromatic fraction from hydroforming is passed through valved line l0 into a fractionator ll wherein light ends boiling between about 170 F. and about 270 F., principally benzene, tolueneand saturated hydrocarbons are removed overhead through line l2. thence to condenser l3 and accumulator drum H, from which a portion may be recycled to the upper portion of tower ll through valved line 15 and the remainder removed from the system through valved line it.

A xylene fraction boiling in the range of about 270 F. to about 325 F. is removed from tower ll through valved line I! and is passed into fractionating tower I8, from which a heavy hydroformate fraction boiling above about 310 F. is discharged through valved line is. The overhead from fractionating tower l8 consisting essentially of the isomeric x lenes'and associated saturated hydrocarbons, boiling in the range of about 270 to about 300 F. is passed through line 20 and condenser 2| to reflux accumulator drum 22, whence a portion is recycled to the upper portion of tower l8 through valved line 23 and the remainder is passed through line 24 and either through valved line 25 and cooler 26 into storage tank 21, or through valved line 28 into fractionating tower 29. Tower 23 is a superfractionator, of which one or more may be used, wherein an ortho-xylene concentrate is separated as a bottoms cut and discharged through valved line 30. A mixture of metaand para-xylenes, together with ethylbenzene and saturated hydrocarbons boiling in the range of about 275 F. to about 285 F. passes overhead from tower 29 through line 3| and condenser 32 into reflux accumulator drum 33, whence a portion is recycled to the top of tower 29 as reflux through valved line 34 and the remainder is passed through valved line 35 to storage tank 36. If it is not desired to efiect fractional distillation of the hydroformate prior to extraction with liquid HF and BF:, it may be diverted from line it through valved line 31 into storage tank 38.

Charging stocks for the initial extraction operation may be passed from any of storage tanks 21, 36 and-38 through valved lines 39, 40 or 4|, respectively, into manifold 42 which leads to the lower portion of extraction tower 43.

The feed stocks to the initial extraction operation should be substantially free of water, since water is tenaciously retained by both HF and BFa. Conventional drying procedures may be used to treat the feed stocks.

Various specific extraction cases will be discussed hereinafter.

Case 1 [Total aromatic cut from hydroforming charged] In this case the aromatic fraction is passed from storage tank 38 through valved line 4| and manifold 42 into the lower portion of extraction tower 43. We prefer to employ a feed containing substantially no components boiling above about 310 F. If desired, a low boiling diluent may be introduced into manifold 42 through valved line 44 in amounts between about 0.1 and about 5 volumes (preferably about 0.8 to 1.5 volumes) per volume of charging stock. Suitable diluents comprise low boiling saturated hydrocarbons such as nor isobutane, pentanes, hexanes, heptanes, octanes; low boiling cycloparafiinic hydrocarbons such as cyclopentane, methyland dimethylcyclopentanes, cyclohexane, methyl-cyclohexane; and the like. The extraction in tower 43 is conducted in such a manner'that two immiscible phases are present therein, viz., a lower extract phase whose upper surface is indicated by meniscus 45 and a supernatant railinate phase 46 above meniscus 45. Boron trifluoride and liquid hydrogen fluoride are introduced into the upper portion of the extraction tower through valved 13 lines 41 and 48, respectively. Part or all of the diluent may be introduced (by lines not shown) directly into the extract phase below meniscus 45 in tower 43.

Although we prefer to employ essentially anhydrous hydrogen fluoride, i. e., HF containing not more than 1 to 2 weight percent of water, we may employ HF containing up to about 5 to 10 weight percent of water. By HF as used herein we intend to denote the molecular species having the molecular weight of 20, which weight is employed in the necessary calculations.

Extraction tower 43 is provided with a manifold 49 to permit the introduction of BFs at various levels in the extraction tower. All or a portion of the HF and/ or BF: may also be added to the charging stock at a pointpr points in advance of the extraction tower 43 and additional HF or BF: can then be added as described above.

The extraction operations in tower 43 are conducted in such a manner as to effect the removal of substantially all the xylenes from the charging stock into the extract phase. In order to secure this result it is necessary to contact the charging stock with a molar excess of liquid hydrogen fluoride (usually between about 5 and about 100 mols or even more of liquid hydrogen fluoride per mol of xylenes contained in the charging stock) and BF3 in the amount of at least 1 mol per mol of xylenes contained in the charging stock. Usually we employ about 1.25 to 2 mols or even more, e. g. 3 or 5 mols, of BF3 per mol of xylenes contained in the charging stock in order to insure complete removal of xylenes from the charging stock into the extract phase. Where the charging stock contains ethylbenzene in addition to the xylenes and where it is desired to remove diethylbenzenes and ethylxylenes formed by the disproportionation of ethylbenzene into the extract phase, an additional quantity of BF; over the amount required simply for the extraction of xylenes is necessary. This additional quantity of BFa is at least 0.5 mol per mol of ethylbenzenes contained in the charging stock.

The temperature employed in extraction tower 43 will ordinarily be maintained between about and about 150 F., preferably 40 to 100 F.; the pressure will be at least suflicient to maintain liquid phase operating conditions.

The charging stock passes upwardly through tower 43 against a counterflow of liquid hydrogen fluoride and BFa, traversing a bed of packing material 50 comprising shaped solid fragments resistant to the action of HF and BFa, for example, carbon, copper or monel metal. Since the extraction is accompanied by considerable release of heat, as pointed out above, a cooling coil is provided to remove all or at least a substantial part of the heat of extraction (heat due to complex formation). If desired the cooling in the extraction operation may be so conducted as to provide an ascending temperature gradient, for example about F. to F. or even as much as F. across the length of the extraction zone.

In the extraction operation a raftinate is formed which consists essentially of the diluent which was introduced through line 44, saturated hydrocarbons derived from the charging stock, benzene and toluene present in the charging stock, benzene derived by disproportionation of some of the ethylbenzene or higher monoalkylbenzenes such as isopropylbenzene, and traces of ethylbenzene which was present in the charging stock and which escaped disproportionation.

.14 The rafflnate may be discharged from the extraction tower and from the system through valved line 52; however, it is usually desirable to pass all or at least a substantial proportion of the raflinate through valved line 53 and heater 54 into stripping tower 55 provided with reboiler coil 55. Tower 55 is ordinarily operated at a top temperature between about F. and about 250 F. and a bottom temperature between about 250 F. and about 400 F. and a pressure between about 0 and about 100 p. s. i. g. to effect stripping of HF, BF: and lowboiling saturated hydrocarbons. The HF, BF; and low boiling diluent passes overhead through line 51 and condenser 58 into a settling and accumulating drum 59, whence BF3 is discharged as a gas through valved line 80, diluent is removed through valved side-line BI and liquid HF saturated with BFa is removed through valved line 62. The BF'3 and'HF solution of BF3 are recycled to the extraction operation, and all or part of the diluent leaving drum 59 through line 6| may likewise be recycled to the extraction operation. When a charging stock containing saturated hydrocarbons is employed, as where a crude hydroformate fraction is employed, saturated hydrocarbons from the rafiinate phase maybe passed through line 63 to valved line 64 back to the extraction zone to operate as a diluent and stripping medium. In this mode of operation, saturated hydrocarbons tend to accumulate in the raffinate and to do away with the necessity of adding a separate diluent through valved line 44. Part or all of the raffinate hydrocarbons may be discharged from the system through valved line 65.

Benzene and toluene can be recovered from sat urated hydrocarbons by extraction or extractive distillation with various solvents such as phenol, ethylene glycol, methanol, nitrobenzene, furfural, nitromethane, liquid sulfur dioxide, and the like. Also, benzene can be separated from other raffinate hydrocarbons by fractional freezing.

The primary extract phase is withdrawn from the bottom of tower 43 through line 66 and is passed through line 68 into a heater 69 repre-' sented in the form of a coil 10 within a shell H through which a heating medium such as steam or other hot gases or liquids may be circulated.

After passing through heater 69, the extract may be treated in either of two ways, viz., all the HF and BF3 may be stripped therefrom and the extracted hydrocarbons may then be subjected to selective extraction with liquid HF and BF's to separate individual xylene concentrates, or only a part of the BF: may be removed from the extract phase, sufficient to lower the BF3 concentration thereof to less than 1 mol per mol of contained xylenes, thus permitting the formation of a second extract phase and a second raflinate phase upon settling, centrifuging or the like. Each processing method will be described in detail hereinafter, taking first the alternative of stripping substantially all of the HF and BFs from the primary extract phase.

The primary extract phase is passed through valved line 12 into manifold 13 and thence to stripping tower 14 provided with reboiler coil 15.

Stripping tower 14 is operated at a temperature between about 100 and about F. and a pressure between about 0 and about 20 p. s. i. g. Lowboiling saturated hydrocarbons capable of forming azeotropes with HF, e. g., propane, butanes pentanes, etc., may be introduced into stripper 14 to facilitate the stripping operation. Suflil cient heat is appliedto the charge to tower I4 by heater and reboiler 15 to efiect substantlally complete removal of HF and BF: overhead through valved line 16, whence these materials may be recycled to the extraction system.

The stripped, extracted aromatic hydrocarbons leave tower 14 through line 11, whence they may be passed through valve I8, line 18 and heat exchanger 80 into selective extraction tower 8|. Since ethylbenzene contained in the charging stock disproportionates to produce C aromatic hydrocarbons which are preferentially soluble in liquid hydrogen fluoride and BFa and since it is usually undesirable to burden the selective extraction carried out in tower 8| with these C10 aromatic hydrocarbons, it is ordinarily desired to divert all-or at least a substantial proportion of the stripped extracted aromatic hydrocarbons from line 11 through valved line 82 into fractionating tower 83, whence C10 aromatics are removed as a bottoms cut through valved line 84 and a mixture of xylenes is taken overhead through line 85 and condenser 86 to an accumulator drum 81, whence a portion may be recycled to fractionating tower 83 through valved line 88 and the remainder passed through valved line 88 and heat exchanger 80 into selective extraction tower 8|. A controlled amount of a low boiling saturated hydrocarbon diluent may be introduced with the charge to extraction tower 8| through valved line 90.

The extraction operation in tower 8| is conducted with a molar excess of liquid hydrogen fluoride, as in the operation of tower 43, but less than I mol of BFa is employed per mol of xylenes charged to tower 8|, resulting in selective extraction of the isomeric xylenes as demonstrated by the different individual xylene distributions in the raflinate and extract phases produced in tower 8|. Tower 8|, like tower 43, is provided with a bed of packing materials 9|, a cooling coil 92, 4 lines 83 and 94 for the introduction of BF: and HF, respectively, into the upper portion of the tower and a BF: manifold 95 for. introducing BFE; at various points along the length of the tower.

In the extraction tower the xylene charging stock iscontacted with a counterfiow of a molal excess of liquid hydrogen fluoride (between about 5 and about 50 mols or even more per mol of xylenes contained in said charging stock) and BE; (in an amount between about 0.2 and about 0.8 mol per mol of xylenes contained in said charging stock) at a temperature between about 0 and about 150 F. under pressure sufilcient at least to maintainthe liquid phase, for a period of time sufiicientto eifect selective extraction, usually between about 1 and about 30 minutes. Preferred operating conditions involve the use of about 7 to mols of liquid hydrogen fluoride per mol of xylenes, about 0.4 to 0.7 mols of BFa per mol of xylenes, a temperature between about 60 F. and about 100 F. and an extraction period varying from about 5 to about 15 minutes.

The raflinate phase derived from the selective extraction operation is characterized by a greatly reduced content of meta-xylene as compared stantiall 16 250 F. and about 400 F. and a pressure between about 0 and about 100 p. s. i. g. to remove suball of the relatively small amounts of HF and F3 carried from tower 8| by the raffinate phase. When a diluent'is employed in tower 8|, it, too, will be stripped from the raffinate in tower 98. The distillate passes overhead through line I00 and partial condenser IOI into an accumulator and settling drum I02, from which BF: is discharged overhead through valved line I03" A stratum oi diluent, is withdrawn through valved side line I04 and liquid hydrogen fluoride saturated with BF: is discharged through valved line I05. It will be obvious that all the eflluents of drum I02 may be reemployed in the various extraction operations.

Stripped raflinate hydrocarbons are discharged from tower 98 through valved line I06 whence they pass through heat exchanger 91, cooler I01 and line I08 to an accumulator drum I09. A portion 01 the raifinate xylenes may be recycled to extraction tower 8| through valved line H0 and the remainder is passed through valved line I I I into fractionating tower I I2.

A para-xylene concentrate is removed from tower II 2 as a distillate through line 3 and condenser M4 to an accumulator drum I I5 whence a portion may be recycled to the upper section of tower II2 as reflux through valved line 6 and the remainder discharged from the system through valved line I II. An ortho-xylene concentrate is removed as a bottoms fraction from tower II2 through valved line II8.

We prefer to employ a sumcient number of selective extraction stages to be able to produce ortho-, metaand para-xylene concentrates, respectively, of at least about 80 mol percent concentration.

An extract phase which is greatly enriched in meta-xylene, based on the distribution of xylenes in the charging stock, is withdrawn from tower 8| through line H9 and is passed through valved lines I2I and I22, heat exchanger I23 and line I24 into stripping tower I25. This tower may be operated under substantially the same conditions as rafllnate stripper 98. Hydrogen fluoride and BF: vapors are removed overhead 'from stripper I25 through valved line I28 for reuse in our process as desired. A portion of the stripped extracted xylenes '(essentially a meta-xylene concentrate) is withdrawn from stripper I 25 through valved line I21, heat exchanger I23 and cooler I28 for recycle to the lower portion of extraction tower 8| to serve as a backwash or reflux. The meta-xylene backwash exerts a stripping effect on the extract phase, apparently by selectively displacing orthoand para-xylenes from their complexes with BF: and HF, resulting in the rejection of orthoand para-xylenes to the raflinate phase in tower 8|. All or the remainder of the meta-xylene concentrate may be withdrawn from the system through with the meta-xylene content of the charging valved line I29.

If C10 aromatic hydrocarbons were not removed from the charging stock to tower 8| in fractionating tower 83', a predominant proportion of these will be retained in the meta-xylene concentrate derived from the selective extraction process and may be removed, if desired, by diverting the concentrate from stripper I 25 through valved line I30 into fractionating tower I3I. By the appropriate choice of operating conditions in tower I3I a meta-xylene concentrate is removed as the distillate through line I32. whence it is passed through condenser I38 into an accumuwhence it passes through a heater I30 of the same type as heater 39,-which was described above. In heater I33, sufficient heat is imparted to the extract phase to render possible the vaporization of part of the BF: contained therein. The heated extract phase is passed through valved line I40 into a settling drum I provided with a line I42 and a pressure control valve I43 for the removal of Blb. Sufilcient BF: is released through valve I43 to reduce its amount in the solution charged to the settling drum to 1 less than 1 mol per mol of contained xylenes, preferably to reduce the BF; content of said solution to a value between about 0.75 and about 0.9 mol per mol 01' contained xylenes. Following the removal of BFa, the solution in settling drum I4I stratifles into two immiscible liquid layers, viz., a raflinate which flows over weir I44 and a secondary extract layer which collects in the lower portion of settler I4I, whence it is withdrawn through valved line I45, following which it may be passed through valved line I46 through heat exchanger I23 and line I24 into stripper I25, whose operation has previously been described, in order to complete the removal of HF and BFa from the extract phase. The raflinate layer which accumulates in drum MI is discharged through valved line I41 into line I43 and thence into stripper 90 (whose operation has previously been described) to effect the removal of small amounts of occluded HF and Bit, following which the stripped raflinate is treated in the same equipment and manner as the rafllnate which leaves extraction tower 3I through line 96, in order to effect its separation into orthoand para-xylene concentrates, respectively.

The above description has been concerned with the method of operation in which a solution of xylenes and other aromatics in liquid hydrogen fluoride and BF:; derived from the extraction, for example, of a hydroformate is stripped of its entire content of HF and BFa, following which the extracted aromatic hydrocarbons are subjected to selective extraction with liquid hydrogen fluoride and BFa to produce individual xylene concentrates.

Consideration will be given at this point to a second method of operation in which the primary solution of xylenes and other aromatics in liquid hydrogen fluoride and BF: derived from the extraction, for example, of a hydroformate is treated to withdraw only a portion of the BF; therefrom in order to reduce the concentration of BF: in the solution to a value between about 0.5 and about 0.9 mol per mol of contained aromatics, preferably to a value between about 0.6 and about 0.75 mol per mol of contained aromatics in order to produce secondary raflinate and extract phases.

Upon the partial withdrawal of BF: from the solution of xylenes and other aromatics in liquid HF and BFa, the solution separates into two immiscible liquid layers, via, a secondary ramnate and a secondary extract, of which the former is relatively rich in orthoand para-xylenes and the latter is essentially a concentrate of metaxylene in liquid hydrogenfluoride and are. It appears thus, that partial withdrawal of B1: from a homogeneous solution of xylenes in liquid HF and BF: produces an effect which is substantially identical with that obtained upon the extraction of a mixture of isomeric xylenes with liquid hydrogen fluoride and an amount of BF; which is less than 1 mol per mol of xylenes charged to the extraction process.

In the second method of operation, the solution of xylenes and other aromatic hydrocarbons in liquid hydrogen fluoride and BF: preparedinfrom the lower extraction tower 43 is removed end of said tower through line 35, thence through valved line and heater 3, through valved line- I49 into a settling and separating drum I50 provided with line I5I and valve I52 for the withdrawal of BFa. Generally, temperatures between about 100 and about 160 F. and pressures between about 0 and about '15 p. s. i. g. are maintained in drum I50. Preferred operating conditions in drum I50 are temperatures between about 120 F. and about 150 F. and pressures between about 15 and about 60 p. s. i. g. Sufficient BF; is withdrawn through line I5I and valve I52 to reduce the BF; concentration in the solution charged to drum I50 to a value substantially below 1 mol'per mol of aromatics contained in said solution, for example, to 0.5 to about 0.9 mol B1 5, preferably to 0.6 to 0.8 mol BFa, per mol of aromatic hydrocarbons. Following the withdrawal of BF: from the solution, it separates into a secondary raflinate phase which is relatively rich in orthoand para-xylenes; the secondary rafllnate phase flows over weir I53 and is removed from drum I50 through valved line I54, whence it is passed through heater I55 into line I43,

thence into stripper 03 (whose operation has been described above) in order to strip HF, BF: and

any low boiling saturated hydrocarbons contained in said secondary rafllnate; A low boiling saturated hydrocarbon diluent, e. g., a pentane,

' may be introduced into drum I50 (by a line not shown) to facilitate the separation process therein. Thereafter the stripped secondary raflinate is processed in the equipment and by the method described in detail for thetreatment of the raflinate passing overhead from selective extraction tower 3| through valved line 05, resulting finally in the separation of said secondary railinate into orthoand para-xylene concentrates, respectively.

The'secondary extract layer formed in the lower portion of drum I50 is discharged through valved line I55, whence it may be passedthrough valved line I51, heater I53 and line I59 into line I24 and HF-BF: stripper I25,-to be processed in the same equipment and manner as heretofore described for the stripping and fractionation (in tower I3I) of the extract phase produced in tower 8I, resulting in the production of a metaxylene concentrate (preferably having a concentration of atv least about volume percent) which may be discharged from the system through valved lines I29 or I30 and a C10 aromatic fraction which may be discharged from the system through valved line I31. If desired, at least a portion of the meta-xylene concentrate may be recycled from lines I29and/or I33 to the lower portion of drum I50 to serve as a backwash.

It will be noted that each of the above-described processes, when applied to the treatment of a mixture of aromatic and saturated hydrocarbon boiling between about 170 and about 300 'F., such as may be produced by the catalytic hydroforming of naphtha fractions, results in the separation of the charging stock into a fraction containing benzene and/or toluene, and saturated hydrocarbons; individual concentrates of ortho-, metaand para-xylenes, respectively; a C aromatic hydrocarbon fraction consisting essentially of diethylbenzenes and ethylxylenes, (predominantly of meta-configuration) produced by disproportionation of ethylbenzene contained in the charging stock.

[0rtho-, metaand para-xylene charged] The method of operation with this charging stock is essentially the same as in Case 1. However, the charging stock (from tank 21) in this case has been prefractionated to remove benzene, toluene and other hydrocarbons boiling below about 270 F. The raiilnate phase passing overhead from tower 43 through valved line-53 will, therefore, not contain these low-boiling hydrocarbons and the stripping operation in tower 55 will, therefore, yield a bottoms fraction, passing through line 63, consisting essentially of saturated hydrocarbons boiling between about 270 and about 300 or 325 F. The composition of the extract phase in tower 43 will be essentially the same as in Case 1 and can be treated in the same fashion to resolve it into its constituents.

Case 3 [Metaand para-xylenes charged] The charging stock is removed from storage tank 38 through valved line 40 into manifold 42 and thence to extraction tower 43 for treatment in the same manner as the charging stocks in Cases 1 and 2. The ortho-xylene concentration in the charging stock is reduced to a relatively low value by prefractionation in one or more superfractionators 29. However, especially where saturated hydrocarbons are also present in die charging stock, some ortho-xylene will be present in the meta-para-xylene concentrate in storage tank 36. The method of treatment of this charging stock is essentially the same as in Cases 1 and 2 except that it may not be considered worthwhile to fractionate a small amount of ortho-xylene from the para-xylene concentrate, produced in the selective extraction or partial BF: withdrawal operations, by the use of tower 2.

A great many chemical reactions, uses and possible applications of the isomeric xylenes have been studied more or less extensively (Xylene Technical Review, published by Oronite Chemical 00., 1947). At the present time it appears that the largest scale demand for orthoxylene is for its employment as a charging stock for vapor phase catalytic oxidation processes for the production of phthalic anhydride (I. E. Levine, Trans. Am. Inst. Chem. Eng. '(Chem. Eng. Progress) 43, 4 (April, 1947), pp. 168-171).

Para-xylene .is desirable as a high octane aviation gasoline component. Another outlet ofpotentially greater commercial significance for para-xylene lies in its oxidation to terephthalic acid which reacts with ethylene glycol to produce synthetic resins (Terylenes) which can be used for the production of synthetic textile fibers (Oil and Gas J., Feb. 5, 1948, page 99) Xylene does not have as attractive commercial outlets as the other two xylene isomers. Any process by which meta-xylene or a meta-xylene concentrate could be converted in high yield into orthoand para-xylenes would be of substantial economic significance. Y

, We have observed that liquid hydrogen fluoride and BEE.- function as highly desirable catalysts to isomerize meta-xylene to equilibrium proportions of the three isomeric xylenes. The isomate or equilibrium mixture of the isomeric xylenes may be subjected to selective extraction with liquid hydrogen fluoride and BF: by the methods described above to produce concentrates of the individual xylene isomers. By the combination of HF-BFa catalyzed isomerization of metaxylene concentrates and the selective separation or concentration of individual xylene isomers by the employment of liquid HF and BFa, it is possible to convert meta-xylene essentially quantitatively into orthoand para-xylenes.

Sufliciently severe meta-xylene isomerization conditions can be selected to induce also the disproportionation of a substantial proportion of meta-xylene to produce toluene and trimethylbenzenes, both of which are in very great demand, so that their production from meta-xylene would yield a substantial profit.

The following is an example which is adduced for illustrative purposes only. The apparatus employed was a 1570 cc. carbon steel autoclave fitted with a 1725 r. p. m. mechanical stirrer. A 258 g. sample of a meta-xylene concentrate, 290 cc. of liquid hydrogen fluoride and 47 g. of BF: were stirred in the reactor at 250 F. for 25 minutes. At this temperature the reactor pressure was 270 p. s. i. g. The reactor was then cooled to F. and the HF and BF: were removed from the reaction mixture by vacuum distillation. The hydrocarbon product, which amounted to 96 percent by weight of the charging stock, was fractionated in a column packed with wire gauze equivalent to 30 theoretical plates and was found to have the following composition:

Vol. M01 Pel- Per Cent Cent Toluene 22 28 xylenes 47 47 Trimethylbenzehes 31 27 Moi Per Cent Feed 65: Equilibrium l ortho-xylene meta-xylene--.

Ema-X lcne. thyl nzene--.-.

I F. D. Rossini, Report on Chemical Thermodynamic Pro rtiea Of the in vi l Xylene isomers. t m tfl- 75 olHydrocarbons, A. El. Research Project, Mar. 31,1947; F .3a.

The above results show that isomerization equilibrium is readily reached and that it is possible by combining the HF-BF: extraction and isomerization operations to reach as near to 100 percent ultimate yield of orthoand para-xylenes as is desired.

Reference will be made once more to Figure 8 to describe in more detail the meta-xylene isomerization and extraction process. Isomerization zone I60 is schematically represented as a tubular reactor surrounded by shell through which a thermophoric medium may be circulated to maintain the desired temperature. It will be obvious that other batch or continuous reactors such as autoclaves, towers, etc. such as are well known in the art of hydrocarbon conversion can be employed in lieu of the tubular reactor. The charging stock is meta-xylene or a meta-xylene concentrate, for example, the material passing through valved lines I29 or I66, which may be passed to isomerization zone I60 (by lines not shown) or meta-xylene concentrates dissolved in liquid hydrogen fluoride and BF: which may be passed from selective extraction tower 6| through valved lines II9, IZI, I13, I62 and I63 into isomerization zone I60; a meta-xylene concentrate dissolved in liquid HF and BF; from settler I through valved lines I45, |6| and I62 to manifold I63, thence to isomerization zone I60; a metaxylene concentrate dissolved in liquid hydrogen fluoride and BF: from settling drum I50, which may be passed through valved lines I56 and I64 into manifold I63 and isomerization zone I60. Additional HF and BF: catalyst may be supplied to isomerization zone I60 through valved line I65 or (by means of lines not shown) at various points along the length of the tubular reactor.

In isomerization zone I60 the meta-xylene concentrate is contacted with liquid hydrogen fluoride in an amount between about 3 and about 20 mols per moi of xylene and BF; in an amount between about 0.05 and about 0.3 mol per mol of xylene at temperatures between about 160 and about 600 F., preferably at temperatures between about 175 F. and about 300 F. for a period of time suflicient to effect substantial isomerization of meta-xylene, usually between about 2 minutes and about 4 hours to allow the isomerization of meta-xylene to equilibrium proportions of ortho-,metaand para-xylenes. The isomerization reaction mixture is discharged from isomerization zone I60 through valved line I66 to be treated to separate concentrates of ortho-,metaand para-xylenes, respectively, and such incidental products as toluene, trimethylbenzenes,

valved line I66 into line I61, thence through valved line I66, heater I66 into manifold 16 and stripper 14 (whose operation has previously been described). Hydrogen fluoride and BF: are substantially completely removed from the isomerization product stream, which is discharged from stripper 14 through line 11 and may be passed through line 19 to extraction tower 6| wherein selective extraction of the mixture of isomeric xylene is conducted as has been described above. If desired, the mixture in line I66 may be passed (by a line not shown) directly into extractor 6|. Alternatively, the mixture of xylene isomers may pass from stripper 14 through lines 11 and 62 into fractionator 66 to remove C9 and C10 aromatic hydrocarbons as a bottoms fraction tower not shown) to separate a xylenes heart out which may be subjected to extraction with liquid hydrogen fluoride and BF: in tower 6|, or the entire product in the accumulator drum may be passed into extraction tower 6|. When 1 appreciable amounts of benzene and toluene are present in the xylene charging stock to tower 6| they will pass overhead as raflinate through line 66 into stripper 66, in which the operating conditions may be adjusted so as to take HF, BFa, low boiling saturated hydrocarbon diluent, benzene and toluene overhead and a xylenes bottom fraction will be discharged through valved line I06 for treatment as hereinbefore described. There are obvious alternative methods for fractionating the rafflnate from tower 6| which will, no doubt, suggest themselves to one skilled in the art and need not, therefore, be detailed here.

Another method of treating the isomerization reaction products involves passing them from valved line I66 through valved line I10, thence through line 66, heater 66 and valved line I46 into settling and separating drum I60. If desired, part or all ofthe isomerization product stream may be diverted from line I66 through line I61, valved line Ill and heat exchanger I12 into line I49 and settling and separating drum I50. The operation of drum I50 has previously been described. In this drum a portion of the BF: is withdrawn from the solution containing xylenes, HF and BFa, resulting in separation of the solution into a raflinate phase relatively enriched in orthoand para-xylenes and an extract phase relatively rich in meta-xylene which can be withdrawn through valved lines I56, I64 and I66 for recycle to isomerization zone I60 to effect the ultimate conversion of substantially all the metaxylene to orthoand para-xylenes.

Although Figure 3 describes a continuous extraction operation, the process of our invention can be operated batchwise. It can also be operated in a plurality 'of stages employing either batch or continuous extraction equipment. Various contacting equipment may be used in lieu of the packed towers shown in Figure 3, for example, agitated autoclaves or the like in combination with settling drums, or a pipe coil wherein the HF, BF: and charging stock are concurrently contacted at high velocity and discharged into a settling chamber. The extraction and phase separation may be conducted in a centrifuge. The above and other known means of contacting employed in solvent extraction processes and in processes wherein liquid catalysts are contacted with hydrocarbons may be employed to practice the. process of our invention.

In order to produce a meta-xylene concentrate having the highest proportion of meta-xylene, we prefer, in one-stage batch operation, to add an amount of BF; just sufiicient to cause the formation of two liquid phases. This amount of Bib will be less than 1 mol per mol of metaxylene contained in the feed stock.

Although HF and especially BF: are relatively expensive reagents and would of necessity be recovered in any large commercial application of the process of this invention, in small scale operations the recovery of these reagents might I be considered immaterial. When the recovery of amass:

HF and BF: as such is not required, the distillation and stripping operations illustrated in Figure 3 may be dispensed with. As an alternative to distillation of the extract phase, said phase may be mixed with water, alkaline or acid solu- 5 tions, or the like which dissolve the HF and BFa, leaving a supernatant hydrocarbon phase which may then be recovered. A desirable acidic solution with which to treat the extract phase is the azeotropic H'F-HaO solution, which becomes enriched in HF upon contact with the extract phase and from which the HF in excess of the azeotropic amount can thereafter be readily recovered by distillation. An alternative would be to distill free HF from the extract phase, following which the residual firmly bound BFa-I-lF-xylene complex could be treated with water or aqueous alkaline or acidic solutions to liberate xylenes bound in said complex. In place of the aqueous solutions mentioned above, one may em- .ploy organic compounds capable of forming complexes with HF and BF: and which are capable of displacing xylenes, especially meta-xylene I from its complexes with HI and BFa, e. g., organic compounds having a more basic (greater electron-donating) capacity than the xylenes, especially meta-xylene. Such organic compounds include various amines, sulfur compounds, e. g., alkyl bisthioethers, and oxygenated organic compounds such as phenol, alkyl ethers, ketones, aldehydes, etc.

Having thus described our invention, what we claim is:

l. A process for the concentration 01' individual xylene isomers from a" solution of at least two isomeric xylenes in liquid hydrogen fluoride and BFa, said solution containing at least 1 mol of BF: per mol of contained xylenes, which process comprises withdrawing BF: from said solution in an amount suflicient to reduce the concentration of BF: in said solution to a value between about 0.5 and about 0.9 mol per mol'oi. aromatic hydrocarbons originally present in said solution and to convert said solution into two immiscible liquid layers, one liquid layer consisting essentially of xylenes in a ratio different from their ratio in said solution and the other liquid layer consisting essentially of a second solution of xylenes in liquid hydrogen fluoride and Bit, the ratio of xylenes in said second solution being different from the ratio of xylenes in the original solution, and separating said liquid layers from each other.

2. A process for the concentration of individual xylene isomers from a solution of metaxylene and at least one additional isomeric xylene in liquid HF and BFs, said solution containing at least 1 mol of BF: per mol of contained xylenes, which process comprises withdrawing BF: from said solution in an amount, suflicient to reduce the concentration 01' BF: in

' said solution to a value between about 0.5 and about 0.9 mol per mol oi. aromatic hydrocarbons originally present in said solution and to convert said solution into two immiscible liquid layers,

one liquid layer consisting essentially of xylenes of substantially reduced meta-xylene content, and the other liquid layer consisting essentially of a second solution of xylenes in liquid hydrogen fluoride and BF'a, said second solution being substantially enriched in meta-xylene in compari on with the original solution, and separating said liquid layers from each other.

3. A proces" for the concentration of individual xylene isomers from a solution of metaxylene and at least one additional isomeric xylene in liquid HF and BFa, said solution containing at least 1 mol 0! BF: per mol of contained xylenes, which process comprises withdrawing BF: from said solution in an amount suillcient to reduce the concentration of BF: in said solution to a value between about 0.6 and about 0.75 mol per mol of aromatic hydrocarbons originally present in said solution and to convert said solution into two immiscible liquid layers. one liquid layerv consisting essentially of xylenes of substantially reduced meta-xylene content, and the other liquid layer consisting essentially 01. a second solution 01' xylenes in is liquid hydrogen fluoride and BE, said second solution being substantially enriched in metaxylene in comparison with the original solution, and separating said liquid layers from each other.

4. 'A process .ior the concentration of individual xylene isomers from a solution consisting essentially of meta-xylene, at least one additional isomeric xylene and at least one Cm aromatic hydrocarbon selected from the class consisting oi diethylbenzenes, ethylxylenes and mixtures of diethylbenzenes and ethylxylenes in liquid hydrogen fluoride and BFJ, said solution containing at least 1 mol of BF; per mol of dissolved aromatic hydrocarbons, which process comprises withdrawing BF; irom said solution in amount sufllcient at least to reduce the concentration of BF: in said solution to a value between about 0.5 and about 0.9 mol per mol of aromatic hydrocarbons originally present in said solution and to convert said solution into two immiscible liquid layers, one liquid layer consisting essentially of xylenes, of substantially reduced metaxylene content and the other liquid layer consisting essentially of a second solution of xylenes and at least one Cm aromatic hydrocarbon selected from the class consisting of diethylbenzenes, ethylxylenes and mixtures of diethylbenzenes and ethylxylenes in liquid hydrogen fluoride and BFa, said second solution being substantially enriched in meta-xylene in comparison with the original solution, separating said liquid ilayers from each other, removing substantially all the hydrogen fluoride and BF; from said second solution, and fractionally distilling hydrocarbons thus derived from said second solution to separate, respectively, a Ca aromatic hydrocarbon fraction enriched in meta-xylene and a C10 aromatic hydrocarbon fraction.

5. A process for the concentration ot individual xylene isomers from a solution of metaxylene and at least one additional isomeric xylene in liquid HF and BFs, said solution containing at least 1 mol of BF: per mol of contained xylenes, which process comprises withdrawing BF: irom said solution in a withdrawal zone in an amount suificient to reduce 'the concentration oi BF: insaid solution to a value between about 0.5 and about 0.9 mol' per mol of aromatic hydrocarbons originally present in said solution and to convert said solution into two immiscible liquid layers, one liquid layer consisting essentially of xylenes of substantially reduced meta xylene content, and the other'liquid layer consisting essentially of a second solution of xylenes in liquid hydrogen fluoride and BFJ,

said second solution being substantially enriched in meta-xylene in comparison with the original solution. removing substantially all the hydrogen fluoride and BF: from said second solu- 7 tion to produce a meta-xylene concentrate and recycling at least a portion of said meta-xylene I concentrate to said withdrawal zone.

6. The process of claim 1 which includes the additional step of distilling the hydrogen fluoride and BF: contained in said second solution, thereby recovering xylenes from said second solution.

7. The process of claim 1 wherein the withdrawal of BF: is effected in the presence of a low boiling saturated hydrocarbon diluent.

8. A process for the concentration of individual xylene isomers from a solution 01' metaxylene and at least one additional isomeric xylene in liquid hydrogen fluoride and BFa, said solution containing .at least one mol oi BF; per mol of contained xylenes, which process comprises vaporizing BF; from said solution at a temperature between about 100 F. and about 160 F. and a pressure between about 0 and about 75 p. s. i. g., continuing the BF: vaporization process until the BF: concentration in said solution is reduced to a value between about 0.5 and about 0.9 mol per mol oi aromatic hydrocarbons originally present in said solution, thereby to 26 convert said solution into two immiscible liquid layers, one liquid layer consisting essentially of xylenes of substantially reduced metaienecontent and the other liquid layer consisting essentially of a second solution of xylenes in liquid hydrogen fluoride and IBFs, said second solution being substantially enriched in meta-xylene in comparison with the original solution, separating said liquid layers from each other and recovering xylenes from said second solution.

10 ARTHUR P. LIEN.

DAVID A. McCAULAY,

REFERENCES CITED 15 The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 20 2,396,965 Passino Mar. 19, 1946 2,397,495 Lien et al. Apr. 2, 1946 2,425,559 Passino et al. Aug. 12, 1947 2,430,516 Lien et al. Nov. 11, 1947 

1. A PROCESS FOR THE CONCENTRATION OF INDIVIDUAL XYLENE ISOMERS FROM A SOLUTION OF AT LEAST TWO ISOMERIC XYLENES IN LIQUID HYDROGEN FLUORIDE AND BF3, SAID SOLUTION CONTAINING AT LEAST 1 MOL OF BF3 PER MOL OF CONTAINED XYLENES, WHICH PROCESS COMPRISES WITHDRAWING BF3 FROM SAID SOLUTION IN AN AMOUNT SUFFICIENT TO REDUCE THE CONCENTRATION OF BF3 IN SAID SOLUTION TO A VALUE BETWEEN ABOUT 0.5 AND ABOUT 0.9 MOL PER MOL OF AROMATIC HYDROCARBONS ORIGINALLY PRESENT IN SAID SOLUTION AND TO CONVERT SAID SOLUTION INTO TWO IMMISCRIBLE LIQUID LAYERS, ONE LIQUID LAYER CONSISTING ESSENTIALLY OF XYLENES IN A RATIO DIFFERENT FROM THEIR RATIO IN SAID SOLUTION AND THE OTHER LIQUID LAYER CONSISTING ESSENTIALLY OF A SECOND SOLUTION OF XYLENES IN LIQUID HYDROGEN FLUORIDE AND BF3, THE RATIO OF XYLENES IN SAID SECOND SOLUTION BEING DIFFERENT FROM THE RATIO OF XYLENES IN THE ORIGINAL SOLUTION, AND SEPARATING SAID LIQUID LAYERS FROM EACH OTHER. 